1. Field of the Invention
The present invention relates to a read-only memory which provides a security circuit built in itself.
2. Description of the Related Art
The read-only memory is abbreviated as ROM later. Herein, the term ROM includes a Programmable ROM.
In order to prevent a program or data written in a ROM from being stolen by an adversary, a security circuit has been traditionally provided between a central processing unit (abbreviated as CPU) and the ROM. The security circuit serves to code-convert select signals sent from the CPU to the ROM such as an address signal, a chip enable signal CE, an output enable signal OE and a chip select signal CS, or to code-convert the data output read out of the ROM. The writing order of data onto the ROM is changed according to the code conversion system given by the security circuit.
The foregoing security remedy, however, needs a considerable time for designing a security circuit. When designing the system having a ROM built in itself, it further requires a designer to consider both the system and the security circuit for designing the overall program. It results in disadvantageously consuming a long time in handling the additional function such as a security circuit and more often creating defective circuit arrangement and program. Hence, more burden is given to the designer.
As another disadvantage, the overall system becomes more costly, because the addition of an additional function such as a security circuit makes the overall circuit more large-scaled, and troubles may more often take place about the circuit arrangement and program when changing the specifications of the security system.
As one remedy for solving those disadvantages, a general-purpose security integrated circuit can be considered. The content of such a security integrated circuit may be made public for a short period. Hence, the circuit has a shortcoming that it requires the system specification to be very often re-designed for security protection.